The present invention is directed to an aluminum alloy and its methods of making and use, and especially to an aluminum alloy having controlled amounts of iron, manganese, chromium, and titanium and controlled levels of zinc for corrosion resistance, particularly resistance to intergranular corrosion.
In the prior art, a number of corrosion resistant aluminum alloys have been developed for use in round and flat tubing applications such as heat exchangers, especially condensers. Some of these alloys are described in U.S. Pat. Nos. 5,906,689 and 5,976,278, both to Sircar, each herein incorporated in their entirety by reference.
U.S. Pat. No. 5,906,689 (the ""689 patent) discloses an aluminum alloy employing amounts of manganese, titanium, low levels of copper, and zinc.
U.S. Pat. No. 5,976,278 (the ""278 patent) discloses an aluminum alloy having controlled amounts of manganese, zirconium, zinc, low levels of copper, and titanium. The ""278 patent differs in several aspects from the ""689 patent, including exemplifying higher levels of manganese, and the use of zirconium.
Both of these patents are designed to produce corrosion resistant aluminum alloys via chemistry control. One reason for better corrosion resistance in the alloy of the ""689 patent is reducing the amount of the intermetallic Fe3Al, as is found in prior art alloys such as AA3102. However, while corrosion is improved, this alloy has a reduced number of intermetallics, and can lack the necessary formability in certain applications, e.g., in the manufacture of heat exchanger assemblies.
The alloys of the ""278 patent can also lack formability in certain instances as a result of the presence of needle-like intermetallics that are generally MnAl6.
In response to these shortcomings, improved aluminum alloys have been proposed in U.S. Pat. 6,458,224 filed on May 3, 2000, which is based on provisional application No. 60/171,598 filed on Dec. 23, 1999, and U.S. Pat. 6,503,446 filed on Jul. 13, 2000. In these improved alloys, the distribution of intermetallics is improved and the intermetallic particle chemistry is controlled for improved formability, corrosion resistance, hot workability, and brazeability. These alloys also exhibit a fine grain structure in the worked product, particularly in alloys employing thin wall structures such as flat or multivoid tubing. By increasing the number of grains via a fine grain size, the grain path becomes more tortuous, and corrosion along the grain boundary is impeded.
However, these improved aluminum alloys still have shortcomings in terms of increased die wear and increased working pressures. In certain applications, the alloys exhibit high flow stresses, extrusion becomes more difficult, and extrusion die wear is increased.
While these improved aluminum alloys do exhibit excellent corrosion resistance under SWAAT conditions, intergranular corrosion at the grain boundaries is still a predominant corrosion mechanism, and corrosion can be a problem in spite of the preferred intermetallic particle chemistry, and fine grain size. Intergranular corrosion can be particularly troublesome once the tubing is brazed together with fin stock in a condenser assembly or the like. First, the assembly of the tubing and fin stock can create a galvanic cell due to the potential difference between the fin stock of one composition and the tubing having another composition, and galvanic corrosion can occur. Second, the corrosion potential difference between certain fin stocks and the tubing can be significant, and in these instances, a tubing that is particularly susceptible to intergranular corrosion can quickly degrade. Such degradation can result in premature failure of the assembled device. This problem can be especially troublesome when tubing is thin walled tubing, e.g., micro-multivoid condenser tubing. With thin wall thicknesses and an intergranular corrosion mechanism, galvanic corrosion along the grain boundaries, can compromise the wall integrity to the point wherein the tubing fails, and the entire condenser assembly must be replaced.
Another problem with these improved alloys is that in some instances, the worked or extruded product must be further cold worked or stretched to meet product dimensional limitations. This added cold work imparts a higher stored energy in the matrix of the material, and this extra energy manifests itself as enlarged grains during a subsequent brazing cycle. Consequently, even though these materials are designed to have a fine grain size to control intergranular corrosion, producing a fine grain size in the pre-brazed product does not always assure that the material will have adequate corrosion protection in its final assembled state.
In light of these problems, a need exists to provide aluminum alloys with improved corrosion resistance and less sensitivity to grain size. The present invention solves this need by providing an aluminum alloy that employs controlled amounts of iron, manganese, chromium, and titanium whereby the electrolytic potential of the grain boundaries fairly matches that of the matrix material, and preferential corrosion along the grain boundaries is minimized. This matching of potentials affords strong protection in situations even where galvanic corrosion is present, i.e., the grain boundaries do not corrode preferentially with respect to the matrix material, and the material corrodes in a more homogenous manner.
It is a first object of the present invention to provide an improved aluminum alloy that exhibits excellent corrosion resistance, does not have intergranular corrosion as its principle corrosion mechanism, and is less sensitive to fine grain size requirements for corrosion control.
Another object of the invention is to provide an aluminum alloy utilizing controlled amounts or levels of iron, manganese, chromium, zinc, and titanium.
One other object of the invention is a method of using the aluminum alloys as components in brazing applications, whereby the similar electrochemical potentials of the matrix and grain boundaries of the components minimize corrosion along the grain boundaries, particularly in situations where galvanic corrosion may be present. The components can be sheet, tubing, or the like.
Yet another object of the invention is a method of making an aluminum alloy wherein a ratio of manganese to iron, a ratio of chromium to titanium, and zinc levels are controlled during the making step to reduce the susceptibility of the alloy to corrosion along the grain boundaries when put in use.
Other objects and advantages of the present invention will become apparent as a description thereof proceeds.
In satisfaction of the foregoing objects and advantages, the present invention is an improvement in long life aluminum alloys using low levels of copper, and manganese, iron, zinc, titanium, and zirconium as alloying elements for corrosion resistance, brazeability, formability, and hot workability. The inventive aluminum alloy consists essentially of, in weight percent:
between about 0.05 and 0.5% silicon;
an amount of iron between about 0.05% and up to 1.0%;
an amount of manganese up to about 2.0%;
less than 0.1% zinc;
up to about 0.10% magnesium;
up to about 0.10% nickel;
up to about 0.5% copper;
between about 0.03 and 0.50% chromium;
between about 0.03 and 0.35% titanium;
with the balance aluminum and inevitable impurities;
wherein the manganese to iron ratio is maintained between about 2.0 and about 6.0, and the amounts of chromium and titanium are controlled so that a ratio of chromium to titanium ranges between 0.25 and 2.0.
In more preferred embodiments, the alloy composition can vary in terms of the amounts of manganese, iron, chromium, titanium, levels of copper and zinc as follows:
The titanium amount can range between about 0.06 and 0.30%, more preferably between about 0.08 and 0.25%. The chromium amount ranges between about 0.06 and 0.30%, more preferably between about 0.08 and 0.25%. The zinc levels can be less than 0.06%, and the ratio of chromium to titanium can range between about 0.5 and 1.5.
The invention also entails the use of the alloy in brazing applications, particularly as part of the manufacture of heat exchanger assemblies. The alloy is particularly effective in assemblies wherein the alloy is employed as tubing, either round, flat or the like, and is brazed to dissimilar materials such as fin stock, headers, or other heat exchanger components.
In making the alloy, the composition is controlled so that each of the manganese to iron amounts and the chromium and titanium amounts are adjusted within the claimed ratios.
The alloy composition can be made into any article using conventional processing of casting, homogenizing, hot/cold working, heat treating, aging, finishing operations and the like. The articles can be used in combination with other articles or components as well.